Hydraulic architecture, spatial variation in light and stomatal conductance of tree branches

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Pinus taeda (08/02/2015, Kew Gardens, London)

Effects of hydraulic architecture and spatial variation in light on mean stomatal conductance of tree branches and crowns

by Ewers B. E., Oren R., Kim H.-S., Bohrer G., Lai C.-T. (2007)

in Plant, Cell and Environment, 30, Issue 4, April 2007, 483–496 – DOI: 10.1111/j.1365-3040.2007.01636.x – 

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3040.2007.01636.x/full

ABSTRACT

In a Pinus taeda L. (loblolly pine) plantation, we investigated whether the response to vapour pressure deficit (D) of canopy average stomatal conductance (G_S) calculated from sap flux measured in upper and lower branches and main stems follows a hydraulically modelled response based on homeostasis of minimum leaf water potential (Ψ_L).

We tested our approach over a twofold range of leaf area index (L; 2–4 m² m⁻²) created by irrigation, fertilization, and a combination of irrigation and fertilization relative to untreated control.

We found that G_S scaled well from leaf-level porometery [porometry-based stomatal conductance (g_s)] to branch-estimated and main stem-estimated G_S. The scaling from branch- to main stem-estimated G_S required using a 45 min moving average window to extract the diurnal signal from the large high-frequency variation, and utilized a light attenuation model to weigh the contribution of upper and lower branch-estimated G_S. Our analysis further indicated that, regardless of L, lower branch-estimated G_S represented most of the main stem-estimated G_Sin this stand.

We quantified the variability in both upper and lower branch-estimated G_S by calculating the SD of the residuals from a moving average smoothed diurnal.

A light model, which incorporated penumbral effects on vertical distribution of direct light, was employed to estimate the variability in light intensity at each canopy level in order to explain the increasing SD of both upper and lower branch-estimated G_S with light.

The results from the light model showed that the upper limit of the variability in individual branch-estimated G_S could be attributed to incoming light, but not the variation below that upper limit.

A porous medium model of water flow in trees produced a pattern of variation below the upper limit that was consistent with the observed variability in branch-estimated G_S.

Our results indicated that stems acted to buffer leaf- and branch-level variation and might transmit a less-variable water potential signal to the roots.